A light-emitting device having a combination of a semiconductor light-emitting element and a phosphor attracts attention as a next-generation light-emitting device where low energy consumption, compact size, high brightness and wide color gamut are expected, and is actively researched and developed. As the primary light emitted from a light-emitting element, the light having a wavelength in a long wavelength UV to blue region, namely, 380 to 480 nm is usually used. Also proposed is a light converter using various phosphors suited for this application.
Further, in recent years, development of a backlight for a small-sized liquid crystal display (hereinafter, LCD (Liquid Crystal Display)) becomes more competitive. In this field, various methods are proposed, however, a method satisfying both brightness and color gamut (NTSC ratio) has not been found yet.
Currently, as a white light-emitting device, a combination of a blue light-emitting element (peak wavelength: about 450 nm), and a (Y, Gd)3(Al, Ga)5O12 phosphor activated with trivalent cerium that is excited by the blue light and exhibits yellow light emission or a (Sr,Ba,Ca)2SiO4 phosphor activated with divalent europium is mainly used. However, in such a light-emitting device, color gamut (NTSC ratio) is about 65% (CIE1931). On the other hand, in recent years, even in a small-sized LCD, more excellent color gamut is demanded. Under such a background, it is an urgent need to improve the color gamut (NTSC ratio) of a backlight for a small-sized LCD.
For example, Japanese Patent Laying-Open No. 2003-121838 (Patent document 1) focuses on color gamut (NTSC ratio) in LCD. Patent document 1 describes that a backlight light source has a spectrum peak within the range of 505 to 535 nm, and an activator of a green phosphor used for the light source contains either europium, tungsten, tin, antimony or manganese, and also describes that MgGa2O4: Mn, Zn2SiO4: Mn is used as a green phosphor in an example. However, when the peak wavelength of the light-emitting element falls within the range of 430 to 480 nm, the phosphor containing either europium, tungsten, tin, antimony or manganese is not entirely applied. More specifically, in the MgGa2O4:Mn, Zn2SiO4:Mn described in the example of Patent document 1, its luminous efficiency is significantly low with excitation light ranging from 430 to 480 nm.
Further, for example, Japanese Patent Laying-Open No. 2004-287323 (Patent document 2) describes, in addition to a RGB-LED where a red LED chip, a green LED chip and a blue LED chip form one package, a three band fluorescent lamp, a UV light LED+RGB phosphor, an organic EL light source and so on. However, Patent document 2 lacks concrete description concerning a RG phosphor that uses blue light as an excitation source.
On the other hand, a tetravalent manganese-activated fluoro-metalate phosphor is described, for example, in US20060169998A1 (Patent document 3). However, Patent document 3 does not describe a combination with a green phosphor of high efficiency and its high color gamut (NTSC ratio).
Patent document 1: Japanese Patent Laying-Open No. 2003-121838
Patent document 2: Japanese Patent Laying-Open No. 2004-287323
Patent document 3: US20060169998A1